Thermometer tube



Dc. 26, 1950 c, o, FA|RCH|LD 2,535,628

THERMOMETER TUBE Filed June 8, 1946 j a 525' {916. INVENTOR mm 55 OWEN fZ/RCjH/LD ATTORNEY Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE THERMOMETER TUBE Charles Owen Fairchild, St. Albans, N. Y.

Application June 8, 1946, Serial No. 675,399

- 21 Claims. 1

This invention relates to improvements in thermometers, and is especially applicable to the mercury-ln-glass type of thermometer. More particularly the invention relates to improvements in the construction of thermometer tubes.

Thermometers, especially those of the mercuryin-glass type, are frequently difficult to read. In previous attempts to make such thermometers more easy to read, an arcuate stripe of opaque White glass, for example, has been inlaid during the manufacture of the tube as a background against which the mercury thread in the bore should appear as a dark line. Under certain favorable lighting conditions, when the mirror surface of the mercury is not reflecting much light compared with that which difiuses from the front of the white stripe, the mercury does appear comparatively dark and the end of the mercury column can be readily located by the observer. Difilculty arises, however, under other lighting conditions when the mercury surface reflects so much light that it also appears bright and cannot be easily distinguished from the white surface behind it. Furthermore, in case the thermometer tube has been provided with the well-known cylindrical lens front, the observers eye must remain near the axial plane of this lens, and a slight movement of the tube or observer may cause a haphazard change in the appearance of the mercury thread from dark to shiny. This sheen may appear in one eye while the thread appears dark in the other eye, a condition which is at once an annoyance and a hindrance in reading the temperature.

Tubes are frequently so constructed that the bore is not circular in cross section, but is elliptical, with the major axis of the ellipse perpendicular to the line of sight so that the bore will appear larger. The bore is placed at the principal focal line of the cylindrical lens, in order that large magnification may be obtained. When this is done, the field of view through the lens is much smaller than the virtual image of the thread, so that only the central portion of the front side of the ellipse is visible from the position near the axial plane of the lens. This part of the front surface is nearly perpendicular to the axis of sight and hence reflects back out through the lens to the observers eye only the light which has entered within a very narrow angle, and if the face of the observer is dark and is directly in front of the thermometer, the mercury will appear dark, as desired. Difiiculty is encountered, however, if the observers eye is an appreciable distance from the axial plane of the lens so that the sides of the elliptical bore become visible. Under such circumstances, light entering from other angles may be reflected toward the observer by the sides of the bore, whether the bore is filled with mercury or empty. This reflected light will, at the high magnifications used, nearly fill the field of view, and will make it very difficult or impossible to discern the location of the end of the mercury thread. Such reflection is usually referred to in optics as total internal reflection.

Various attempts in the past have been made to remedy this difficulty of total internal reflection from the bore. For example, the field of view has, by various means, been drastically restricted, but such arrangements frequently make it annoyingly difficult to find the direction from which the magnified image of the mercury can be seen at all. Other attempts involve the use of a bore having very sharp edges designed to reduce the possible region from which undesirable reflections may occur. Such a construction causes difliculty, however, in that it is hard to manufacture and also in that droplets of mercury are caused to adhere in the sharp troughs at the sides of the bore. Other unsatisfactory attempts to solve the problem of total internal reflection involve the use of various types of shields in combination with a supposedly light background and dark mercury thread, but such shields invariably reduce the illumination on the background, and hence decrease the contrast between the mercury and the background, and they also produce various other undesirable effects, as a result of improper formation, positioning and nature of the shields.

An object of the present invention is to provide a thermometer tube which is easily readable and which is free from the difliculties enumerated above, as well as other difficulties.

The present invention is applicable to thermometers designed to be read with the eyes of the observer lying in or near the axial plane of a viewing lens, such as clinical thermometers, and is also applicable to thermometers designed to be read with the eyes of the observer in a line approximately perpendicular to the axial plane of the viewing lens, such as wall-type thermometers.

With reference to thermometers designed to be read in the latter manner, an object of this invention is to provide binocular visibility of the whole width of the mercury thread while avoiding undesirable reflections from the edges of the thread.

A feature of the present invention is that the mercury thread appears more consistently bright against a dark background, instead of fortuitously dark against a bright background as in various earlier thermometers, thereby taking advantage of the natural reflective qualities of mercury.

A further feature of the present invention is that although the background appears dark, the level of illumination within the tube, and especially on the front of the mercury thread itself, is high.

A. further feature of the present invention is that the mercury thread may have a tinted pearance with a background tinted with a contrasting color.

A still further feature of the inventionis that the tube may be almost circular in cross section with the bore located near the center thereof. This approximately circular shape is advantageous from the standpoint of ease of manufacture and is alsomore convenient to theuser. example, chemists frequently find that thermometers the cross sectionsof which are ap proximately triangular, or otherwise consid ably different from circular, cannot be cash used in some types of'ap-paratus, such as wher it is desired-to insert thethermonieter through a ircular hole in a'rubberstorpper.

The above-mentioned, as Well as other objects, together with the many advantages obtainable by the practice of the present'invention, will readily comprehe'nded by persons'skilled inthe art by reference to the following detailed description taken in connection with the annexed drawingswhich respectively describe and illu trate preferred embodiments of the invention, and wherein Fig. 1 is a front elevational view of a mercur in-glass thermometer, showing a dark background band above the'mercury.

Fig. 2 is 'a schematic side elevational view of a thermometer as it might be viewed by an observer, the line of sight in this case being directed downwardly from the horizontal.

Figs. 3 and 4 are enlarged cross-sectional views of thermometer tubes illustrating embodiments of the present invention.

Fig. 5 is an enlarged cross-sectional view of a clinical thermometer, also illustrating the teachings of the present invention, the scale of enlargement being greater in this case than in Figs. 3 and 4, since clinical thermcmeters'are in fact normall considerably smaller than other'types of thermometers.

Fig. 6 is a schematic diagram illustrating 'certain optics involved in the present invention.

In Fig. 1 there is generally illustrated a thermometer to which the teachings of'this invention are applicable. Such a thermometer may be provided with a glass bulbfiffilled with mercury 9. Integrally formed with and "communieating with the bulb is a glass tubegener-ally indicated at Ill. Various embodiments will be descrlbed below of'tubes'which may be used in such a thermometer.

The'schematic side elevational View shown in Fig. 2 illustrates'the manner in which athermometer of the Wall type'niay sometimes be viewed by an observer. At a later point, certain disadvantages of previous'thermometers will be pointed out in connection with this manner-of viewing, along withadvantages of thermometers constructed according to the present-invention.

Reference is made to-"Fig. 3 which showsyin enlarged cross section, one embodiment of a thermometer tube illustrating the teachings of the .present invention. The tube IE is provided with portions forming a cylindrical viewing lens ii, and a pair of cylindrical, auxiliary, lightgathering lenses [2, l2. It is to be understood that the term cylindrical is used in this application with its broad meaning, applying to any curved surface generated by the motion of a straight line parallel to itself and constantly interseoting a curve. It may be noted that this curve need'not'necessarilybe circular. While the lenses in the various illustrated embodiments "may satisfactorily have cross sectional contours which are circular arcs, the present invention is not limited to such configurations.

The tubeli is provided with a lengthwis bore [3, located in the axial plane of the viewing lens i l and closer to this lens than the principal focal line of this lens. The position of this focal line is indicated by the numeral id. The bor it may be shaped in cross section in theformoi an ellipse'with its major axis perpendicularto the axial plane'of the'viewing lens ll. lenses l2,l2 are focused on the bore E3.

The tube isprovided with a stripe or band lii of colored, dark, 'or seler'ztively absorbing'glass. This'stri pe isgenerally parallel to the here it, and in cross section embraces'samefromthe rear, that is, from'the direction opposite the viewing lens, through a subtendedangle of more than 180 and-less than 210, orpreferably ap proximately 208. Thus'the stripe idin'ayextend in a somewhat circumferential =manner partly around the bore, so 'as tc subtend attlie boreapproximately 20 more than'therearhalf -of the tube, that is, 10 beyond exactiy half, at each end. Outside the stripe'lfi, which-in the present example may beblack, is a stripe'lt of lighter white, diffusing glass, which prevents light which has entered the tube at various points frombeing largely absorbed by the dark stripe. Thus this light-colored stripe 'raiseslthe level of illumination within the tube. The dark stripe serves both as a background i'nthe line of sight past the bore and as a shield of 'thesar'ne shade 'as'this background'to'be refieoted'b'y the empty bore in the major'lportions of the zones of total reflection. Thus, ifthere were no such shield,lig'ht striking the "bore from a direction corresponding to one end of the crescent-like cross-sectionof the dark stripe wouldbe reflected out the lens"! I, causingthe empty bore toappear bright. Inasmuch as the 'dark stripe is present, however, any reflections of light comingfrom this direction appeardarl -and merge with the dark background. Inasmuch as the: dark stripe does not extend'very far forward of the bore, it permits the bore .to be illuminated froin thcfront so that themeroury filled-bore appears-bright, as desired,.-against the dark background. The ends or edge reg-ions" l6a, lda 'of the-whitestripe are extended in thinner section forward of the dark stripe-so as to transmit and scatter light received from various directions toward the bore onits'forward side. There is providedin front of-and close 'to-the bore ita thin, diffusing partiall 1 transparent stripe or screen I lpwhich transmits and scatters toward=the mercury surface light received from all forward directions, and acts likewise on light reflected from that surface, so that'the 'mercury-filled bore appears brightfrom all directionsagainst the dark background. Thelig'ht-gathering lenses |2,-i2,.being focusedon the bore, concentrate ilig'ht .iupon .it

and help to cause the mercury thread to appear bright.

Without the stripe I! or the portions [Ea of stripe IE or both, if the observer is looking at the thermometer in a manner such as that 11- lustrated in Fig. 2, a dark object might be clearly reflected in the mercury, causing the mercury to appear dark and hence indistinguishable from the dark background. Certain previous attempts to construct thermometer tubes with dark backgrounds have had disadvantages of this nature. By the means described, however, this difficulty has been overcome in the present invention.

It is to be understood that of the light from the front striking the screen ll only a small fraction is scattered toward the front and that where this screen is backed by the empty bore most of the light scattered toward the back is absorbed by the dark stripe 15. Where the screen ll is backed by the mirror surface of the mercury the bore appears to glow, and in fact it is not required that screen I! completely destroy the mirror-like or specular reflection from this surface.

Opal, milk or white glasses used as screen I! for scatterin or diffusing light do not in general scatter light of all colors equally. Hence the mercury thread behind this screen may ap-- pear tinted or colored. This is not objectionable, and may be enhanced by deliberate addition of coloring matter of the same hue to the glass from which the screen II is formed, or a neutralizing color may be added to whiten the light issuing from the front of the screen I]. If this light is tinted or colored, for example, red or orange, the stripe !5 can be colored green or blue, instead of black, to selectively absorb light which has passed toward the stripe'through screen I! and the empty bore. If the screen I1 is not tinted, the main body of the tube In may be tinted or colored instead. While the combination described is particularly adaptable to the employment of color contrast between the mercury-filled and empty bore, it should be kept in mind that where the tube is illuminated by white light or ordinary daylight, color contrast is obtained only with considerable sacrifice of brightness contrast.

The simple optics involved with the bore and the viewin lens H is illustrated in Fig. 6. In this figure there is shown in schematic cross section a cylindrical lens surface 3! l, similar to the lens H, and an elliptical bore 3|3 similar to the bore 13. The focal line 3! of the lens 3H is located to the left of the bore (H3. The diagram shows the position of the virtual image 323 of the bore 333, and it is noted that in this illustration the magnification is rather limited, being approximately three times.

In the drawings the lenses are shown as having circular cross-sectional contours, although, as stated, other configurations could be used satisfactorily. The center of curvature of the lens 3 is shown at 324.

It is observed that with the focal line 3M of the lens 3H located to the left of the bore 313 the rays from said bore emerging toward the right from the lens diverge in a wedge. If the angle between the sides of this wedge is greater than approximately 14, binocular visibility of the bore is possible with the eyes of the observer in a line perpendicular to the axial plane of the lens and symmetrically spaced on either side of this plane.

This angle is based upon the average spacing between the eyes of humans, about 2.5 inches, and the average near point distance of normal vision, of about 10 inches.

Reference may be made to Fig. 4,. which i1- lustrates a somewhat different embodiment. In this figure the tube is indicated generally by the numeral H0, and is provided with a viewing lens Ill and a pair of light-gathering lenses I I2, H2, similar to the lenses H and l2, 12 of Fig. 3. There is provided a bore H3, the cross-section of which may have the shape of a distorted ellipse, to be described in more detail below. The relative positions of the focal lines of the lenses with respect to the bore are similar in Fig. 4

- to those in Fig. 3.

There is provided a light-absorbing stripe H5, corresponding to the stripe l5 of Fig. 3. Outside this stripe is a light stripe H6, having ends HBa, HBa, which extend to include the subtended angles of the lenses H2, H2, respectively, and which are drastically thinned, in contrast to the shape of the ends lfia, Iiia shown in Fig. 3; There is no stripe in the embodiment of Fig. 4 corresponding to the stripe or screen I! of Fig. 3.

The ends Ha, H6a serve to diffuse and scatter light toward the viewing side of the bore, especially light entering through the lenses H2, H2.

The bore H3 has fiat sides respectively facing approximately half-way between the viewing lens H! and the light-gathering lenses H2, H2. Stated differently, the flat sides are respectively approximately perpendicular to the planes which bisect the angles between the axial plane of the viewing lens and the respective axial planes of the light-gathering lenses. The fiat sides are joined at the front by a curved surface, which may be of shape similar to the ends of the ellipse. Where the front surface of the bore is perpendicular to the line of sight, there will upon occasion appear a dark stripe as a result of reflection of a dark object, but this will not influence the visibility of the mercury thread and its contrast with the background because this stripe includes only a small fraction of the visible width of the bore. In this connection it is to be remembered that the bore is not at the principal focus where the magnification would be in the neighborhood of times, but forward of the focus where the magnification will be suflicient to make the Virtual image comparable in size with the window of the lens or even smaller if the bore I I3 is particularly small. Obviously, the dark stripe on the mercury, mentioned above, will not be as dark as it would be were the front of the mercury thread not well illuminated, and will in any case be lighter than the background. The statements about colors and coloring in connection with stripes I5, l6 and I? in Fig. 3 apply in Fig. 4, in connection with stripes H5, H5 and the drastically thinned ends use, Hfia of stripe H6.

Fig. 5 shows the cross section of a tube of a clinical thermometer on a still larger scale, these thermometers usually being smaller than others. This embodiment is provided with an elliptical bore H3 and with two lenses 2! 8 and 219 either of which can be used as a viewing lens while the other permits light to reach the mercury thread. It is customary to read a clinical thermometer with both eyes on or near the axial plane of the viewing lens and of course it is improper to at tempt to read such a thermometer from such a position as that illustrated in Fig. 2. If the lens nearest to the observer is used, light from the assesses 7 eeil'ing of the ro'om, "or rrom -a window;mayxenter the other liens. lf' theobserver is r a nurse .in a white ddr ess -she "may look th'rough I the farther lens iwhile light :froml'her dress :enters itheinear lens, :making :the amerc'ury Ithread :appear a :fiat whitexagainstithesdarklbackground. Stripcs 215, and 1216 with the sends rzliia, 5.21% :are disposed similarlyito-thosefshown iniFigrB. .There is providedrallightistripeim ofathe samea'glassias that of ith'e stripe :2 [S and subtendingitheangle Ibetween'i th'eilenses :2 i 8 :and 1-2 i 9, and serving :as an additional diffusing :means which has its aparticularedutywhen the? main sourcerofllight is at onesideinfithetobserver. :Lenses :2 l8wands2 l 9' are not focused on the bore but arexfocused beyond the boreon' lines 22 [3217661222, like lens I l. in Fig. 3, sozthat in :Fig. 5, ithebore will .be magnified for example l-n-r times insteadwof the three times shown in Fig. 3Wh=ichgenerally sufilces for ordinary zlarge ibores. The bore in Fig.6, 'although smallerithaniin the type Of thermometer shown in.Figs.'3 and 4, d0es' not have to be highly magnified because it is well illuminated and being in front'oflthaprincipalfocus of the viewing lens,

its image can-be found by an observer-whose eyes are "wellraway 'frointhe -axial planeof the lens, inrthe diverging'wedge .o f reflected light issuing from the lens. This diverging Wedge need not have an angle of 14* or more as would berequired for binocular observation With'the observers eyes on opposite sides of the -axial plane of the lens, because, as-stated,- the clinical thermometer is always'iread with'both' eyesin this plane, or-near itonthe same side--'but anangleor ,ifor example -is very helpful as an aid to finding the line of sight. Having'this aid a novice needs none of the painstaking instructlon required for reading clinical thermometers having the conventional white back andwith'abore at the principal focus Whereit is-visible through an angle of 1 or less.

In the-various embodiments it is preferable that each =lenssubtend an'ang-le of'less than 50-at the bore, since this-will result in-a more nearly round tube. It is also preferable that the aggregate subtended angle iofthe =lenses be greater than since this-results in: gathering and directing tow-ard't-he mercury'ia greater amount-of light, especially when'the' observer .is'on the viewing aXisof one'lens.

'It is obvious-that reflections from the outer surface of the thermometer tubacould-be practical-ly' eliminated by the now Well known means for nan-glare treatment, thus furtheriimproving thelgeneral readability of the thermometer.

vIt maybe pointed out that a narrowbright line, such as that correspondingto' themercurythread in the present thermometer, is inherently easier tolseethan a narrow dark line, and is comparable invisibility with the broader, supposedly dark line corresponding to the mercury'thread'i-n certain previous thermometers.

It can be seen that forthe various abOvereasons, the presentinvention' provides a thermometer tube which, in addition to other advantages,

'gives'improved contrast between the appearance Ofthe empty bore and that of the filled'part,'re-

gardless of the lighting conditions and relative positions of the thermometer and observer.

'Whiletheillustrative-forms of the invention have been disclosedin considerable detail, it will be understood that various changes maybe made in the construction and=arrangement of the severalpartswvithout departing from the general principles-and scope of'the invention as defined by the apperided claims,

Jrclairn:

1. A glass thermometer tube having ael'ongitudinalsboraa viewing lens oni thei 'frontcof :said tubeifocusediinithe.rearlof said iibore at leastcone auxiliary lengiadj acen't said viewing: lens: focused on said .bore,gailongitudinalsdark stripe-embedded iniisaidlitube embracing inore .than Lthe rear half oissaid borega;longitudinal.lightsstripe embedded in; said tube 1. outsidessaid dark sstripe embracing said here .on all sides iexceptjn i the angle 5511bt-ended byssaiddensesaat-zsaidlbore, and :a thin diffusing stripe Lpfil'alll to said bore Lsubtendmg at said :bore ;at.:1eastthe angle of said ;.v-iewing lens. v

i. 2. 4A ither-mometer tube having a i longitudinal bore Sand :a "plurality of :cylindrical :.lenses, :s-at least onerof said lensesnhaving.itszprincipalliocal iinerlocated beyond sa-idtborefiromsaid one-Jens, and. atileastrone of .said:.-lenses having its :prirr cipal focal.lincisubstantiallysat said 1 bore.

3. A thermometer :tubernf slightly distorted circular cross :section i'havinguai longitudinal: bore and arpluralityofgcylindrical lenses, at; least one or" said lenses having its principal focal .line' located beyond said bore froin'said one. lens, said core being in cross ;section of the general :shape of an ellipse :With I its maj or 1 axis perpendicular to :the.:aXial plane'zoff-saidaonelens, atleast one ofisaidlenses having its prin ipal. focal: line substantially atsaid bore.

4. A thermometer tube having 4 a longitudinal bore, a plurality of -;cylindrical :lenses 1021 the front side of said tube,'at .-least one of saidlenses being focused beyond- -said bore, :and "arelative- 1y rise-shield stripeparallel to and behind. said bore,:.subtending-in crosssectionmore than 180 and less than 210.

: 5. A thermometer etube having a longitudinal here, :a plurality .of cylindrical lenses on athe frontside of-said tube, atleast one-cfsaid lenses being ,a viewing-lens and having .its principal focal line displaced from saidborein a direction opposite said -.one lens, "and a relatively .dense shield stripeparallelito andbehind-said :boro comprising-a 'background therefor and subtendingin .crosssection suflicientlymore than the rearhalf thereof to-reduoe substantiallytotal internal reflection of light byssaid borethrough said viewinglens.

'6. A thermometer tube havingv a longitudinal bore, said .tubeib'eing fshapedlto form plurality of parallel a'djacentdongitudinal lenses on .the front :of said tube, at" least one of said lenses" being "focused beyond said bore, "a longitudinal dark stripe of arcuate cross section behind and generally parallel to "said l'oore, subtending an angle 'of 'isubstantially 200 at said bore, .and .a 'light ."sti'ipe of 'a'rcuate "cross section behind "and generally 'parallel to said dark stripe, said light *stripe'eXte-nding partly around said bore outside said dark stripe "and heyond each longitudinal edge of said dark stripe sons to subtcnd a-greater-angle at said bore than does" said dark" stripe.

'7. A thermometer tube SilfiDEdtWfOlll'l a plurality of =para-llel *aidjacentlongitudinal lenses on the-front of said tube, -said tube having a longitudinal b'orepatleast one of said lenses having its 's'cipal 'i ocal line located 'beyond said bore *fromsaid one lens, --and at-least one of said lenses having its principal focal'line sub stantial-lyat said boreyalongitudinal darkstripe of 1 arcuate across section 1 behind 'an'd generally parallel-to said-bore, subtending an angle-of =more than l I and lessthan 210" *at said-bore,

and a light stripe of arcuate cross section behind and generally parallel to said dark stripe, said light stripe extending partly around said bore outside said dark stripe and beyond each longitudinal edge of said dark stripe so as to subtend a greater angle at said bore than does said dark stripe.

S. A thermometer tube having a longitudinal bore, a viewing lens on the front of said tube focused in the rear of said bore, an auxiliary lens on each side of said viewing lens focused on said bore, a longitudinal dark stripe embedded in said tube partly embracing said bore from the rear, and a longitudinal light stripe embedded in said tube outside of said dark stripe embracing said bore on all sides except in the angle subtended by said viewing lens at said bore.

9. A thermometer tube having a longitudinal bore, a viewing lens on the front of said tube focused in the rear of said bore, an auxiliary lens on each side of said viewing lens focused on said bore, a longitudinal dark stripe embedded in said tube partly embracing said bore from the rear, and a longitudinal light stripe embedde in said tube outside said dark stripe embracing said bore on all sides except in the angle subtended by said viewing lens at said bore, said light stripe being so thin in the regions adjacent said auxiliary lenses that it transmits toward the bore a major fraction of the light converging toward said thin regions from the auxiliary lenses.

10. A glass thermometer tube having a capillary bore and at least one lens front having its principal focus at such a distance in the rear of said bore that the light reflected from said bore emerging from said lens fills a wedge the sides of which diverge at an angle of more than 14.

11. A thermometer tube having a longitudinal bore of approximately elliptical cross section, a dark stripe embracing substantially the rear half of said bore, two lenses extending longitudinally along the front half of said tube each having its axial plane passing approximately through said bore, and each said lens having its principal focal line located farther than said bore from said lens, a light stripe outside said dark stripe partly embracing said bore through the subtended angle extending rearwardly around the bore between the rearmost longitudinal edges of the two 12. A glass thermometer tube having a capillary bore for receiving mercury, a dark stripe parallel to said bore and shielding same on all of its rear side, light-scattering stripe means embedded within said tube adapted to maintain said bore brighter on its forward side than said stripe when said bore is filled with mercury, at any degree of illumination of said tube, for any direction of incident or issuing light, and a viewing lens focused behind said bore.

13. A glass thermometer tube having a capillary bore for receiving mercury, a dark stripe parallel to said bore and shielding same on all of its rear sides, light-scattering stripe means embedded within said tube adapted to maintain said bore brighter on its forward side than said stripe when said bore is filled with mercury, at any degree of illumination of said tube, for any direction of incident or issuing light, and a plurality of lens fronts, the axial planes of which 10 pass through said bore, atleast one of said lens fronts being focused behind said bore.

14. A glass thermometer tube having its outer surface shaped to form a plurality of parallel lenses extending longitudinally along the front half of said tube, at least one of said lenses having its principal focal line on the opposite side of said bore from said one lens, the glass of the main body of said tube being lightly colored to render light transmitted thereby dominant in a predetermined color, and a light-absorbing stripe embedded in said tube embracing approximately the rear half of said bore, said stripe being adapted to absorb selectively light of said 0010!.

15. A thermometer tube having a longitudinal bore, a viewing lens on the front of saidtube having its principal focal line behind said bore, an auxiliary lens on each side of said viewing lens, each of said auxiliary lenses having its principal focal line substantially at said bore, said bore being shaped in cross section in the general form of a distorted ellipse, said ellipse having its major axis perpendicular to the axial plane of the viewing lens and being distorted by having flattened sides respectively perpendicular to the planes which bisect the angles between the axial plane of the viewing lens and the axial planes of the respective auxiliary lenses, a dark stripe embracing said bore from a direction opposite said viewing lens, and light-diffusing means embedded within said tube between said auxiliary lenses and said bore.

16. A thermometer comprising a tube having a bore extending longitudinally therethro-ugh, a bulb communicating with said bore, and mercury in said bulb and a portion of said bore, said tube having a lens front the principal focal line of which is located in the rear of said bore at such a relative distance that the whole width of the virtual image of said bore is visible to an observer whose eyes are in a line at right angles to the axial plane of said lens front and symmetrically spaced from said plane, said line being at a distance of 10 inches from said lens front, said tube also having longitudinal stripe means embedded therein for substantially reducing the visibility of the total internal reflections seen in the side zones of said image.

17. A glass thermometer tube of slightly distorted circular cross section having a generally central longitudinal bore, the outer surface of said tube being shaped to form a plurality of parallel cylindrical lenses extending longitudinally along the front side of said tube, each of said lenses having its axial plane passing through said bore, the distance from at least one of said lenses to its principal focal line being sufficiently greater than that to said bore that the whole width of the virtual image of said bore is visible through said one lens when viewed in a normal manner from points lying in the axial plane of said one lens, and embedded stripe means adapted to reduce substantially the visible internal reflection from said bore.

18. A thermometer tube having a longitudinal bore for receiving mercury, at least one viewing lens focused beyond said bore, embedded lightabsorbing means forming a background against which said bore may be viewed, at least one lightgathering lens, embedded light scattering and diffusing means, and embedded means for substantially reducing the total internal reflection of light by the bore through said viewing lens.

19. A thermometer tube having a longitudinal bore for receiving a column of mercury, at least 1 11 one viewingr; lensfocused behind: said: bore; embedded light-absorbing meansforming; a background; against which said: bore: may; be viewed, atieast one: lig hteg athering" lensfocused ion said bore; embedded light scattering; andti diffusing 5 means; and embedded; means for; substantially reducingihe total internal refiectiomofdight-by the boreathroughwsaidviewing lens; each of; said lenses-:subtending anzangleirof lessthan'n-fiflf at'said bore;:,the: aggregate; angle-subtendedzjat: said Shore by. all saidlenses :beinggreatenthan 180?.

20: A. thermometer-"tube? according gtosclaimx 10, including" alongitudinak stripe embracing said borer from the rear subtending atsaidibore: in cross sectionzanrangle of morerthan 180 andiless than 210?.

2 1. A: thermometer; tube having: a;-: longitudinal bore;ja, lens-1 front focused behind saidzbor amd an longitudinal stripe; embracing: said bore from therrear through an anglegreater than180f.

CHARLES OWENEFAIRCHILD.

REFERENCES CITED The following references 'are of record-in the me of this patent:

UNITED STATES PATENTS Number? Name Date:

278,380 Weinhagen May-29, 1883 310,925: Weinhagen; Jan; 20, 1885 1;5615,925 Hespex Nov. 17, 1925 13630385 Hespei; May 31, 1927 FOREIGN PATENTS Number" Country Date 149,361 Germany Mar. 14,1904 357.25% Germany! A112. 19. 1922 

